Treatment of metals



July 18, 1939.

Oxyda/ian-Va/ues J. M. MICHEL TREATMENT OF METALS Filed Aug. 25, 1938 Oxyoa/izm V lues Empera/ures dosef By His Afforneya M D/chrama/e 700 Hana c/rram ale lnvenioi' Martn Ma'chged Patented July 18, 1939 UNITED STATES PATENT T'JOFFICEJ I TREATMENT OF METALS Josef Martin Michel, Bitterfeld, Germany, assignor to I. G. Farbenindustrie Aktlengesellschaft, Frankfort-on-the-Main, Germany Application August 25, 1938, Serial No. 226,738 In Germany August 28, 1935- 13 Claims. (Cl. 148-131) I This invention relates to improvements in the that the oxidation value of a melt composed of treatment of metals and is a continuation in part sodium dichromate and/or potassium dichromate of my application bearing Serial Number 95,015 asymptotically decreases to' a certain definite and filed in the United States Patent Oflice on value when the melt is left for some considerable August 8, 1936, bearing the title Heat-treatment time at a constant temperature above 400 C. i; of magnesium alloys, and also of my application It has now been ascertained that this decombearing Serial Number 143,908 and filed in the position of potassium dichromate is counteracted, United States Patent Ofllce on May 21, 1937, if the fused saline baths also contain an amount bearing the title Process of improving workpieces of potassium monochromate at least correspondm ade from metals of the iron group and their ing to the constant oxidation value of the melt (in alloys. the absence of monochromate) at the working In the United States Patent specification 2,036,- temperature employed. The monochromate may 563 entitled Heat-treatment of magnesium albe added as such, to the bath, or the desired ratio loys", there is described and claimed a process for of dichromate to monochromate can be obtained the heat-treatment of light metal alloys containy the additiOh o Corresponding quantities of 15 ing magnesium which comprises submerging the salts such as potassium nitrate which are capable alloys in a bath substantially consisting of molten f n f rm n d ch i monochromateanhydrous sodium bichromate or potassium bi- The potassium a s y be p y. y, chromate, or mixtures thereof, the preferred proreplaced y e COITeSpOnding Sodium Sa s. The portion in such mixtures'beingone part of potash accompanying drawing contains two graphs, 20 sium dichromate to three parts of sodium dichrowhich e been drafted on the basis of p ,mate. ments. These graphs make it possible, in a In using such baths for the heat-treatment, at Simple mannerfor a given application e temperatures above 400 1, of workpieces of for ture of a saline bath to ascertain theexact ratio example, aluminium-base alloys containing mag betweeli Sodium bichromate, Potassium b c o- 26 nesium, or of metals of the iron group and their mate, and Sodium monochromate, Potassium alloys, it is found, especially when the b th are monochromate. respectively, to be used in accordkept in use over prolonged periods, that dee ance with the present invention. In view of the position phenomena occur which lead to the depcircumstance, however, that in practical Wo 30 5 111 of sludgy products which have been the prescribed temperature of the bath is occa- 30 shown by investigation to consist chiefly of chro- Sionally exceeded, it s advisable to p y in the mic oxide, whilst the viscosity of the saline bath t, a slightly larger amount of monochromate gradually increases. This decomposition further than would be required to j pe sate or t e sult i a progressive change in t composition ascertained constant oxidation value of the mono- 85 of the bath and also in some cases in the setting Q fi melt, at the temperature 6 36 up of an injurious reaction with the surface of the cemedworkpieces under treatment. This is particularly For a complete understanding of y invention. the case, when the fused saline baths contain, or reference s made t accompanying drawing consist of fused alkali metal dichromates, because in which, I 15 a graph in which oxidation 40 ingreasing temperatures are accompanied by a values are plotted against temperatures and Fig. 40 progressive decomposition of the dichromates, II is a graph in which oxidation values are plotted t t formation of monochromate and chro against the equilibrium ratios of various mixtures mium oxide, in accordance with the equation: s of ImoFnio clhirlomatcg atnd diC11l1Omat3S.l t d th i n g. eox1aionvaues,cacuae on e 4Me2CnO7a4Me2CrO"+2Cn03+302 basis of chromlc acid titre=cubic centimetres of where Me denotes either sodium or potassium, or normalsolution of NazSzOa/l grm. (ordinates), a mixture of both cations. This reaction continare plotted against temperatures (abscissae). ues until a certain ratio between dichromate and Fig. I thus enables one to ascertain the oxidation monochromate has been reached which represents value of a bath for a given temperature simply by a state of equilibrium which varies with the temfollowing the temperature ordinate u t t ihter- 50 perature, in the sense that, fora bath of given sects the curve, and from that point running composition, there is a well-defined partial oxygen along the corresponding abscissa, until the latter pressure corresponding to each temperature strikes the ordinate axis, from which the oxida- This theory regarding the course of the decomtion value corresponding to the given temperature '55 position reaction has b een confirmed by the fact is read.

In Fig. II the equilibrium ratios between baths consisting of N azCrO4 and N2L2C1'20'l Na2CrO4 and KzCrzO'z KzCrO4 and NazCraOr K2CrO4 and KzCrzOr I mates, and from right to left for the monochromates. Taking therefore the oxidation value ascertained for a bath of a given temperature from Fig. I, it is only necessary to follow the parallel to the abscissa axis through the ordinate corresponding to the given oxidation value, and ascertain the abscissac of the points at which that parallel intersects the equilibrium lines, representing the respective binary systems.

Taking for instance a temperature of 500 C., Fig. I gives an oxidation value of 207, and this latter when followed up in Fig. II, shows that both a melt consisting of about 58% NazCrzO7 and 42% NaiCi-Oi, and also a melt consisting of about 769,, NLuClzOv and 24% K2CIO4, will be in equi librium at that temperature.

For an annealing temperature of 650 C. baths of the following composition, for example, are suitable:

Ya) 82 parts of K2CI2O7 and 18 parts of KNOz b) 63 parts of K2C12O'z and 37 parts of KzCIO4 lo) 42 parts of N212C1'2O'1 and 58 parts of KzCrOi The foregoing mixtures have an oxidation value of 185, which is thus about 6 points below the value corresponding to equilibrium at 650 C. so that increases of temperatures to about 700 C. for short periods can be permitted without appreciably altering the composition of the bath.

The content of monochromate thus required is, in the case of temperatures between 400 and 500 C. at least about but preferably slightly higher, for example from 7 to 10%. The effect of this proportion of monochromate in the dichromate baths is that decomposition phenomena, accompanied by the deposition of sludgy products, are prevented from occurring at temperatures above 400 C.

In the preparation of baths in accordance with 'the invention, the requisite amount of monochromate can be added, as such, to the dichromate bath. or the monochromate canbe formed in the bath itself, by adding to the latter suitable amounts of substances, such as caustic alkalis, alkali peroxides, alkali nitrates, and also alkali carbonates, which are adapted to convert dichromate into monochromate in the melt.

The fused saline baths of the present invention exhibit the further advantage that corrosion of the bath containers such as is sometimes observed particularly in the case o iron vessels when using purely dichromate baths, is completely avoided.

The invention will now be illustrated with reference to the following comparative tests:

A mixture of 750 parts by weight of sodium dichromate and 250 parts by weight of potassium dichromate is fused in an iron vessel and then heated to 500 C. Test pieces of aluminium-magnesium alloys are then immersed in the melt, After 24 hours, sludgy deposits already appear in the bath, increasing progressively as time goes on and finally rendering the melt viscous and unsuitable for practical use. The initial oxidation value of 220 cc. of n/IO NazSzOa per 1 grm.-

of the melt had fallen to 208 after 10 days. Analysis of the sludge showed the presence of chromic oxide contaminated with 20% of ferric oxide, resulting from the action of the liberated oxygen on the walls of the vessel.

If, on the other hand, in accordance with the 5 invention, there are added to the mixture of alkali dichromates fused in the foregoing manner,

30 parts by weight of potassium hydrate (90% concentration) corresponding to 23 parts by weight of K20, the oxidation value of 206.5, dc- 10 termined in the melt before use, remains unchanged after 3 weeks service. No deposit is observed.

The same effect can be obtained by an addition of 21 parts by weight of caustic soda.

The same results are obtained by treating the saline mixture, prepared as above, with 50 parts by weight of potassium nitrate and employing the mixture at 500 C. The following oxidation values are obtained:

First day of test: Oxidation value 207.

Twentieth day of test: Oxidation value 207.

The bath functions without any decomposition.

The same effect is likewise obtained when the mixture of 750 parts of sodium dichrornate and 250 parts of potassium dichromate is treated with 100 parts by weight of monochromate (75 Na2CrO4+25 K2CrO4) in which case the oxidation values are:

First day of test: 207.

Twentieth day of test: 206.

In this case also no appreciable deposit is observed.

It-has been observed that the resistance to incidental and subsequent corrosion of workpieces made of numerous metals of the iron group, and of their alloys, is extensively diminished; or practically prevented altogether by the treatment with the fused saline baths according to the invention, as the following summary will show:

Temperature of test: 500 C.

Duration of test: 11 days.

1. Saline melt-750 grms. of NazCrzO7+250 grms. of KzCrzOv (without added stabiliser).

Loss in weight in gmnnnes per sq. metro pcr (lllllll (representing incidental corroslon) Material Chrome-nickel steel. Nickel 2. Saline melt-J grms. of NaCr2O1+250 grms. of K2C12O7+10% of K2C1'O4 (according to the present invention) Lnss in weight in grammes pcr sq. metro pcr lllem (roprcsentin incidentnl sion) M atorlal (.OITU- position, firmly adherent coatings of a colour ranging from dark brown to black, according to the duration of contact, are formed on the workpieces. It has been found that, especially in the pecially suitable as a priming for the subsequent application of coloured lacquers and the like.

The following table shows the influence of a preliminary treatment at 500 C., with fused saline baths according to the invention over various periods on the subsequent corrosion resistance for various commercial metals, the extent of corrosion having been determined by direct measurement of the current strength in a 2% solution of sodium chloride (Todt method). In each case, the reading was taken after immersion for 2 minutes.

Immersion Corrosion in Improve- Material in bath, amperes per ment,

minutes sq. cm. lpercent I. Iron:

(a) Untreated 8.5 to 9.5 (b) Treated in accordance with the invention 2 6.3 30

5 5.9 34.5 5.3 41 4.3 52 40 4.7 48 II. Nickel:

(a) Untreated 1.0 (b) Treated in accordance with the invention 0.6 60 0.3 60 120 0.3 60 III. Monel metal:

(a) Untreated 1.7 (b) Treated in accord ance with the invention 10 0.7 50

I claim:

1. A process for the heat-treatment of metals and alloys included in the group consisting of the metals of the iron group and their alloys, and light metal alloys containing magnesium, which comprises submerging said metals or alloys in a bath, substantially consisting of at least one molten anhydrous compound of the group consisting of sodium bichromate and potassium bichromate, and at least one compound of the group consisting of sodium monochromate and Potassium monochromate, the content of monochromate being at least approximately corresponding to the equilibrium ratio between dichromate and monochromate at the temperature of the heat-treatment.

2. A process for the heat-treatment of metals and alloys included in the group consisting of the metals of the iron group and their alloys, and light metal alloys containing magnesium, which com prises submerging said metals or alloys in a bath substantially consisting of about three parts of anhydrous sodium bichromate and about one part of anhydrous potassium bichromate, and at least tures between 400 and 500 C., which comprises submerging said alloys in a bath, substantially consisting of at least one molten anhydrous compound of the group consisting of sodium bichromate and" potassium bichromate, and at least about 5% by weight of an alkali monochromate.

4. A process for the heat-treatment of lightmetal alloys containing magnesium at temperatures between 400 and 500 C., which comprises submerging said alloys in a bath, substantially consisting of at least one molten anhydrous com.-

' pound of the group consisting of sodium bichromate and potassium bichromate and between about 7% and about 10% by weight of an alkali monochromate.

5. A molten saline bathforthe heat-treatment of metals and alloys included in the group consisting of the metals of the iron group and their alloys, and light metal alloys containing 'magnesium, which bath substantially consists of at least one molten anhydrous compound of the group consisting of sodium bichromate and potassium bichromate, and at least one compound of the group consisting of sodium monochromate and potassium monochromate, the content of monochromate being at least approximately cor-' responding tothe equilibrium ratio between dichromate and monochromate at the temperature at which the bath is to be used, and said bath being substantially free from sludge.

6. A molten saline bath for the heat-treatment of metals and alloys included in the group consisting of the metals of the iron group and their alloys, and light-metal alloys containing magnesium, which bath substantially consists of about three parts of anhydrous sodium bichromate and about one part of anhydrous potassium "bichromate and at least one compound of the group consisting oi sodium monochromate and potassium monochromate, the content of monochromate being at least approximately corresponding to the equilibrium ratio between dichromate and rnonochromate at the temperature at which the bath is to be used, and said bath being substantially free from sludge.

7. A molten saline bath for the heat-treatment of light-metal alloys containing magnesium at temperatures between 400 and 500 0., which. bath substantially consists of at least one molten anhydrous compound of the group consisting of sodium bichromate and potassium bichromate, and at least about 5% by weight of an alkali monochromate, said bath being substantially free from sludge.

8. A molten saline bath for the heat-treatment of light-metal alloys containing magnesium at temperatures between 400 and 500. 0., which bath substantially consists of at least one molten anhydrous compound. of the group consisting of sodium bichromate and potassium bichromate, and between about 71% and about 10% by weight or an alkali monochromate, said bath being substantially free from sludge.

9. A molten saline bath for the heat-treatment of metals and alloys included in the group consisting of the metals of the iron group and their alloys, and light-metal alloys containing mag nesium, substantially consisting of anhydrous dichromates and monochromates, in which the ratio of monochromate to dichromate at least corresponds to the equilibrium ratio between monochromate anddichromate at the temperature of the bath, and said bath being substantially free from sludge.

10. A method of preparing a bath of the ldnd described for the heat-treatment of metals and alloys included in the group consisting of the metals of the iron group and their alloys, and light metal alloys containing magnesium, which comprises adding to a fused bath substantially consisting of alkali dichromates a substance adapted to convert dichromate into monochromate in proportion corresponding at least to the equilibrium ratio between monochromate and dichrornate at the temperature at which the bath is to be used, and said bath being substantially free from sludge.

11. A method of preparing'a bath of the kind described for the heat-treatment of metals and alloys included in the group consisting of the metals'of the iron group and their alloys, and

light metal alloys containing magnesium, which comprises adding to a fused bath substantially consisting of alkali dichromates a substance se-' lected from the group consisting of the caustic alkalis, alkali metal peroxides, alkali metal nitrates, and alkali metal carbonates, in such proportion as to establish, by conversion oi part of amases such dichromates into monochromates, a-ratio between monochromate and dichromate in said bath corresponding at least to the equilibrium ratio between monochromate and dichromate at the temperature at which the bath is to be used,

and said bath being substantially free from sisting of about 63 parts by weight 'of sodium bichromate and about 37 parts of potassium monochromate.

JOSEF MARTlN MICHEL. 

